1. Field of the Invention
The present invention relates to a process for the production of a substrate incorporating semiconductor devices and in particular those permitting a double control system of a flat liquid crystal screen in which the addressing of each elementary point is realized by non-linear amorphous silicon elements and the control of the rows and columns is provided by thin polycrystalline silicon film transistors integrated on to the same substrate around the active part of the screen. At least two types of device are used on the same basic material, namely amorphous silicon obtained by the thermal decomposition of silane. The selective crystallization on the periphery of the active part is carried out by annealing in a furnace having a temperature gradient.
2. Description of the Prior Art
The addressing of matrix access display screens can be carried out in different ways. This addressing can be carried out by transistors deposited in thin film form on e.g. a glass substrate, each transistor being associated in series with an electrooptical material image element. These transistors are preferably produced from amorphous silicon permitting a better homogeneity of the characteristics on large surfaces. The disadvantage of this addressing is due to the difficulty of finely controlling the properties of the different films constituting the thin film transistors. Thus, these transistors are formed from several films of materials such as amorphous silicon, insulants, etc. and it is difficult to obtain good insulants and good interfaces between the different films by low temperature processes. These production difficulties increase with the number of transistors to be implanted and particularly when the number of transistors is equal to the number of image elements of the screen.
Thin amorphous silicon film transistors are also not satisfactory for the integrated control of row and column electrodes, due to their low admissible direct current, which imposes matrix control by circuits outside the screen.
An integration of this control on the basis of thin film transistors produced on polycrystalline layers obtained by laser annealing of amorphous silicon has been the subject of tests. On the basis of this method, it is possible to obtain a display screen, whose image elements are controlled by amorphous silicon transistors and the rows and columns by polycrystalline silicon transistors. However, although the transistors obtained by laser annealing have very good performance characteristics, it is difficult to produce them in large numbers with a good reproducibility. It is therefore necessary for the thermal method used to provide an excellent crystallization uniformity. It is also known to control the image elements by non-linear elements of the varistor type.